Method and apparatus for improving wellbore productivity with piezoelectric crystals

ABSTRACT

A system and method of enhancing production of oil and gas from a reservoir is provided. More specifically, the present invention relates to systems and methods of positioning piezoelectric crystals in a production interval of a well. When activated, the piezoelectric crystals produce displacement (elongation and compaction) and cavitation within the wellbore to move particles within the well and fractures in the reservoir. In one embodiment, a fluid which includes a proppant material and a plurality of piezoelectric crystals is pumped into the well. The fluid may be a hydraulic fracturing fluid. In another embodiment, a plurality of piezoelectric crystals are interconnected to a body of a downhole assembly. The downhole assembly is configured to be positioned within a production interval of a wellbore. In one embodiment, the body includes a hollow bore. In one embodiment, the piezoelectric crystals are interconnected to at least one of an exterior surface of the body and an interior surface of the body within the hollow bore.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority and benefits under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application Ser. No. 62/521,972, entitled“System and Method of Fracture Stimulation and Conductivity Monitoringin Conventional and Unconventional Oil and Gas Production” filed on Jun.19, 2017, which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention generally relates to systems and methods ofenhancing production of oil and gas from reservoirs. More specifically,the present invention relates to systems and methods of positioningpiezoelectric crystals within a production interval of a wellbore. Inone embodiment, the piezoelectric crystals are mixed with proppants in ahydraulic fracturing fluid to be pumped into the wellbore and fractures.Additionally, or alternatively, piezoelectric crystals may beinterconnected to a downhole assembly configured to be placed within theproduction interval of the wellbore.

BACKGROUND

The exploration and development of unconventional and low permeabilityreservoirs plays a significant role in meeting and exceeding the U.S.and global energy needs. However, well production generally declinesover time. In shale gas formations, shale oil formations, tightsandstone formations, and other unconventional formations with lowpermeability, the greatest decline in oil and gas production frequentlyoccurs within 6 months to 1 year from the first production.

Current techniques of mapping fractures in a reservoir cannot accuratelydetect the actual lengths of fractures created during hydraulicfracturing operations. Thus, the effective lengths of fractures detectedusing current fracture mapping techniques may be shorter than the actuallengths of the fractures. One of the reasons for the shorter lengths isimproper placement of the proppants into the fractures. Proper proppantplacement within fractures plays a significant role in maintainingconductivity of the fracture once the hydraulic fracturing treatment iscompleted and production initiated. Inaccurate mapping of fracturelength and width along with the overall geometry of the stimulatedreservoir volume will cause insufficient transport and placement ofproppant into the fractures. The size and shape of the proppantsselected for use as well as the resistance of the proppants to localstress changes and to preventing fracture closure also plays asignificant role in fracture conductivity.

Crosslinked gel fracturing fluids have been used to provide betterproppant suspension. While crosslinked gel fracturing fluids can carryproppants of any size deeper into the fractures due to the higherviscous characteristics, unfortunately, even crosslinked gel fracturingfluids cannot provide satisfactory proppant transport into the fracturesunder downhole reservoir conditions. Further, cleaning the crosslinkedgel fracturing fluids from a wellbore is a significant challenge. Thus,there is often some of the crosslinked gel fracturing fluid whichremains within the proppant pack and within the reservoir resulting indamage to the reservoir. This is particularly problematic whencrosslinked gel fracturing fluid is used in low permeability gasformations.

Using slickwater to transport and place proppants during hydraulicfracturing reduces the damage caused by crosslinked gel fracturingfluids in the reservoir. As will be appreciated by one of skill in theart, slickwater generally refers to a water-based fluid and proppantcombination that has low-viscosity. Chemicals may be added to the waterto increase fluid flow and reduce friction. In addition, biocides,surfactants, and scale inhibitors may be added to the slickwater. Theuse of slickwater is especially beneficial in the hydraulic fracturingof unconventional reservoir formations. Salt ion types of slickwater areparticularly beneficial. Various concentrations of slickwater have beenformulated to make the slickwater fluid compatible with the proppantselected for a formation associated with a wellbore. However, improperproppant placement and settling, banking within the fractures createdduring hydraulic fracturing as well as the compaction of the proppantpack and reduction of the proppant pack porosity and permeability withtime are strongly dependent on the composition of the fracturing fluidbrine. Further, increasing effective stress in the wellbore can crushthe proppants if the stresses exceed the proppant strength. The crushedproppants prevent efficient long term production from the well.

Experiments have been conducted using Niobrara shale core samplesutilizing distilled water, 2% and 6% NaCl and 2% and 6% MgCl₂ solutionsflowing through the fractured formation that was filled with monolayerceramic proppants. The experiments indicated large changes in thestiffness of the formation and an associated reduction of fractureconductivity that was observed occurred faster when stiffness wasreduced in spite of the selection of strong proppant.

Applying light or ultralight proppants can help transport the proppantsfurther into the fractures. However, light and ultralight proppants aretypically more expensive than denser proppants. The light and ultralightproppants also tend to crush faster than more dense proppants in deeper,high in-situ stress conditions. Unfortunately, crushed proppantsintroduce additional debris that can also contribute to formationdamage. Thus, the use of light or ultralight proppants may be associatedwith reduced fracture permeability and a decline in fractured reservoirproduction after a short production time.

Several proppant placement processes have recently been introduced toovercome some of the limitations in the conventional proppant placementtechniques. These processes include hybrid fracturing, reverse hybridfracturing, alternate-slug fracturing, and channel fracturing. Each ofthese techniques are used to increase the conductivity in the proppantpack, providing highly conductive paths for the flow of oil and/or gasfrom the reservoir into the wellbore. These techniques have been shownto achieve better proppant placement within the fractures, includingplacement of proppant into fractures of longer length. However,limitations still exist with the use of these proppant placementprocesses.

Permeability measurements have been conducted in core samples obtainedfrom several conventional and unconventional reservoir formations. Theformations tested include hydraulically fractured Barnett, Eagle Ford,Niobrara and Vaca Muerta shale gas and tight oil formations.Measurements have been conducted under in-situ stress conditions withelevated pore pressure. The samples were first measured in their intactstate, prior to application of hydraulic fractures and the mechanicaland acoustic/ultrasonic properties were recorded. The samples weresubsequently fractured and additional measurements were taken. Aselected fracturing fluid with a specific composition was flowed throughfractures in the samples at different differential stresses. Thepermeability and fracture conductivity measurements indicatedsignificant reductions in conductivity and permeability of the fracturesin the samples of all formations within short periods of time. Theresults are evidence explaining the rapid production decline infractured tight oil and gas shale formations.

Geochemical analysis of the samples after various fluids flowed throughthe fractured rock identified significant rock-proppant-fluidinteractions for specific fluids resulting in dissolution of selectedminerals in the matrix. The interactions and dissolution introduceassociated reduced stiffness in the formation. As will be appreciated byone of skill in the art, the formation stiffness is measured withYoung's modulus. Reductions in formation stiffness cause easier failurethat results in grain crushing. The crushed grains as well as otherfines may subsequently be transported by fluid flow within the proppantpack in the wellbore as well as within the fractures. The migration ofthe fines into the proppant pack blocks pore throats within the proppantpack. The embedment of proppants in fracture faces and the migration offines into the proppant pack results in a reduction of conductivity tothe wellbore and an associated decrease in production from thereservoir.

When the fracture conductivity decline results in an uneconomicalproduction level, one method of the improving fracture conductivity andproduction is refracturing the wellbore. The evaluation of a candidatewell to determine the source of production decline is highly important.The evaluation of the candidate well is also important to determine costand identify a suitable fracturing fluid for continuing the economicallyviable production status in wells formed in tight sandstones, shale gas,tight oil and other unconventional reservoirs. Refracturing may increasethe conductivity and produce an associated increase in production forsome limited time, yet temporarily increasing the production will notcompletely eliminate the production decline. Both hydraulic fracturingand refracturing operations require large amounts of water and chemicaladditives and raise concerns related to contamination of groundwater andsurface water. Further, the cost of transporting the fracturing fluidand proppants to the well are significant. Accordingly, alternatestimulation techniques are needed for economical and environmentallyfriendly production from tight gas sands, shale gas, tight oil andconventional and unconventional oil and gas reservoirs in the U.S. andworldwide.

Removal of reservoir formation damage using ultrasonic energy has beenstudied, and acoustic stimulation tools using elastic waves withfrequencies typically ranging from KHz to MHz range have been found toremove near-wellbore damage, such as skin effect. The mechanisms behindremoval of formation damage using an acoustic streaming approach was notwell known until early 1990's. A study of coupling of mechanical andchemical forces and associated modeling on the equilibrium separationdistance for acoustic wave propagation that lead to acoustic streamingof small particles oscillating parallel and perpendicular to a planewall surface revealed the process of acoustic streaming could beutilized in the oil industry.

The efficiency of acoustic cleaning as a function of frequency, outputpower and the type of fluid interacting with the solid particles hasalso been studied. Experimental and modeling studies have also shownthat ultrasonic cleaning may be a useful stimulation technique in theremoval of near wellbore damage caused by drilling and drill-in fluids.Studies have also been conducted to discover the key parameterscontrolling the efficiency of acoustic cleaning in reservoir conditions.In 2008, one study investigated the effect of fluid type, sonificationtime and applied sonifier power level on the cleaning efficiency of thenear wellbore formation damage caused by fines and solids in drillingand drill-in fluids. In another study, experiments were conducted withdynamic filtration of fully brine saturated reservoir core samples. Anexperimental study of damage removal near a wellbore in a conventionalformation concluded that acoustic cleaning was effective in drillingrelated applications.

The effectiveness of acoustic stimulation of the screens typically usedin frac-pack operations in deepwater applications for production frompoorly consolidated reservoirs has also been studied. The screens areoften plugged by sand and particles migrating from the formation throughfluid flow. This plugging of the screens results in mechanical damage ofthe screen in these high permeability formations thereby reducing thehigh-rate production to levels similar to low permeability formations.Acoustic stimulation experiments and modeling have also been conductedto define the mechanisms of plugging in order to determine how toeffectively unplug the screens. A field tool for acoustic stimulationthat may be used for near wellbore damage removal applications isdescribed in U.S. Pat. No. 7,216,738, which is incorporated herein byreference in its entirety.

Prior art acoustic stimulation tools available in the industry areplaced in the well through wireline or tubing. Because of this, priorart acoustic stimulation tools can only be used to eliminate formationdamage very near the wellbore. Additionally, the effectiveness of thesetools is strongly dependent on the frequency at which the stimulationtools operate. However, all prior art acoustic stimulation tools utilizefrequencies which are effective only up to a few inches from thewellbore. Accordingly, prior art acoustic stimulation tools only cleanareas very near to the wellbore prior to the proppant packing processwhen hydraulic fracturing treatment is implemented in the wells. None ofthese tools generate frequencies which are effective to clean areas morethan a few inches away from the wellbore into the reservoir formation.

Another deficiency of all of the prior art stimulation tools is theirneed for downhole power. The tools can only be used by utilizing powerprovided by a surface connection or by a downhole battery. Downholebatteries limit usage of the stimulation tool to the life of thebattery. Another problem with current stimulation methods and tools isthat in order to apply acoustic cleaning or re-fracturing to improve thepermeability and fracture conductivity, production from the well must bestopped adding further loss of revenue caused by loss of the productionduring the treatment in addition to the operation cost of thecleaning/re-fracturing processes.

Due to these and other limitations associated with known systems andmethods, there is an unmet need for a system and method of transportingand placing proppants in fractures and cleaning a wellbore and theproppant pack in fractures along a production interval of the well.

SUMMARY OF THE INVENTION

The present invention provides systems and methods of cleaning awellbore, placing proppant within the fractures along a productioninterval of a wellbore and/or within the fractures into the reservoirwhich cannot be implemented using the prior art. The methods and systemsof the present invention are more effective than the prior art andwithout some of the costs and disadvantages. One aspect of the presentinvention is a novel method for proppant pack placement and removal ofproduction related damage in the proppant pack and real time monitoringof the fracture conductivity.

One aspect of the present invention is a system and a method ofpositioning piezoelectric crystals within a production interval of awellbore. In one embodiment, the piezoelectric crystals expand andcontract, or move, in response changing conditions within the wellbore.In one embodiment, the movement of the piezoelectric crystals istriggered by changes in one or more of pressure, temperature, and fluidflow within the wellbore. In another embodiment, the movement of thepiezoelectric crystals may be initiated by electricity. The movement ofthe piezoelectric crystals removes particles from proppant packed in thewellbore and fractures in the production interval. More specifically,the movement of the piezoelectric crystals allows particles to migrateout of the proppant pack when the well is flowed. The movement of thepiezoelectric crystals also moves the proppant into the fractures. Inthis manner, the piezoelectric crystals enhance the permeability andflow rates from the wellbore.

Another unique aspect of the invention is that the piezoelectriccrystals are self-actuating. When elongation or contraction of theformation or proppant pack takes place as a result of fluid flow, thepiezoelectric crystals within the fractures and the proppant pack expandand/or contract. As the piezoelectric crystals move in response to theexpansion and/or contraction, the piezoelectric crystals contact eachother and neighboring proppant particles. The resulting vibration anddisplacement create additional drag forces in the proppant pack. Theseforces push the fines into the neighboring piezoelectric crystal and/orproppant particles. In this manner, the piezoelectric crystalsfacilitate movement and proper placement of proppants into the inducedfractures through the hydraulic fracturing treatment and/or naturalfractures that are connected to other natural fractures and hydraulicfractures.

In one embodiment, when there is a reduction in one or more of fluidpressure, temperature, and flow rate, the piezoelectric crystals willcontract. In another embodiment, when there is an increase in one ormore of fluid pressure, temperature, and flow rate, the piezoelectriccrystals will expand or elongate.

It is another aspect of the present invention to provide a hydraulicfracturing fluid which includes a mixture of proppant and piezoelectriccrystals. More specifically, in one embodiment, a predetermined ratio ofpiezoelectric crystals and proppants are mixed together. The hydraulicfracturing fluid with the proppants and piezoelectric crystals maysubsequently be pumped together into the wellbore. When the hydraulicfracturing fluid is pumped into the wellbore and fractures of aformation, the piezoelectric crystals are transported into the fractureswith the proppant. In this manner, the piezoelectric crystals can beplaced in the production interval of the wellbore and into fractures ofthe formation around the production interval. The piezoelectric crystalscan thus be positioned at, or near, the ends of the fractures which arespaced from the wellbore.

The piezoelectric crystals may be substantially evenly distributedwithin the proppant pack as far away from the wellbore as they can bepumped through the combined induced and natural fracture networkintroduced during the hydraulic fracturing treatment. The mixture ofpiezoelectric crystals and proppant is beneficial both during pumpingand during production of the well. Specifically, pressure, temperatureand flow differentials during hydraulic fracturing cause thepiezoelectric crystals to expand and contract. The expansion andcontraction of the piezoelectric crystals helps transport and positionthe proppant within fractures in the formation. Similarly, thepiezoelectric crystals included in the hydraulic fracturing fluid alsoexpand and contract in response to production variations. The expansionand contraction is transferred through the proppant pack to fines withinthe proppant pack. As the well flows, the fines migrate out of theproppant pack and maintain (or improve) fracture conductivity.

It is another aspect of the present invention to provide a downholeassembly which includes piezoelectric crystals. The downhole assembly isconfigured to be placed within the production interval of the wellbore.The downhole assembly has a body to which the piezoelectric crystals areaffixed. Optionally, the body may have a generally cylindrical shape. Inone embodiment, the body is solid. Optionally, the body comprises asteel casing or bar. Alternatively, the body is formed of a mesh orscreen. Optionally, the body includes a bore with an interior surface.In one embodiment, the piezoelectric crystals are interconnected to anexterior surface of the body. Additionally, or alternatively, in anotherembodiment, the piezoelectric crystals are interconnected to theinterior surface of the bore.

Another aspect of the present invention is a method of monitoringfracture conductivity. The method may be used in production ofreservoirs such as tight gas sandstone, shale gas, tight oil, shale oiland other conventional and unconventional formations. In one embodiment,the method includes a 3-D stimulation application as a function of time(i.e., 4D-stimulation) as the method provides real time monitoring ofthe fractures in the reservoir in proximity to the wellbore through theplaced piezoelectric crystals. In one embodiment, the method includesrecording data associated with the activation of one or morepiezoelectric crystals positioned a wellbore and/or fractures within thereservoir. The data may include frequencies generated by thepiezoelectric crystals when activated.

It is another aspect of the present invention to provide a system andmethod of improving fracture conductivity of a wellbore while thewellbore is in production. In one embodiment, the system and methodinclude a downhole assembly that may be positioned within the wellbore.The downhole assembly is configured to be selectively activated whilethe wellbore is in production. In this manner, the production of thereservoir and wellbore can be increased without taking the wellbore outof production.

One aspect of the present invention is a system and method of increasingconductivity of fractures of a production zone proximate to a wellbore.In one embodiment, the system and method includes removing damage in thewellbore. For example, in one embodiment, particles associated withproppant, natural or man-made fines, and completion residue are movedout of the fractures with the system and the method of the presentinvention.

Another aspect of the present invention is a method of increasingfracture conductivity. The method includes positioning piezoelectriccrystals in the proppant pack. In one embodiment, the piezoelectriccrystals are mixed with the proppant in a hydraulic fracturing fluid.The hydraulic fracturing fluid is then pumped into the wellbore andreservoir. Additionally, or alternatively, the piezoelectric crystalscan be interconnected to a downhole assembly of the present invention.In one embodiment, no chemicals are injected into the wellbore with thehydraulic fracturing fluid including that proppant and piezoelectriccrystals. In this manner, the system and method of the present inventioneliminates, or at least reduces, unintended or inadvertent contaminationof ground water and surface water during production from a reservoir.Increasing fracture conductivity without the use of chemicals alsoreduces costs associated with operating the wellbore and isenvironmentally safer than prior art methods of increasing fractureconductivity that include injecting chemicals into the wellbore.

Another aspect of the present invention is a system and method ofcleaning part or all of a proppant pack in fractures of a hydrocarbonreservoir. More specifically, the system and method of the presentinvention may be used to clean fractures in the formation. In oneembodiment, a downhole assembly of the present invention is positionedin the wellbore. Piezoelectric crystals are interconnected to at leastone surface of the downhole tool. Optionally, piezoelectric crystals mayalso be mixed with the proppant before the proppant is pumped into thewellbore. The piezoelectric crystals are subsequently triggered oractivated to clean the proppant pack.

In one embodiment, the piezoelectric crystals are activated throughchanges in pressure, temperature and/or fluid flow within the wellbore.More specifically, in one embodiment, fluctuations in one or more ofpressure, temperature, and fluid flow within the wellbore trigger thepiezoelectric crystals. In one embodiment, the piezoelectric crystals donot require external electric power, such as from a battery or anelectrical wire.

When triggered, the piezoelectric crystals expand and contract. Theexpansion and contraction causes displacement in the neighboringproppant particles as well as other piezoelectric crystals creating achain reaction of self-movement within the proppant pack. In someapplications, at specific frequencies and particularly in gas wells,this movement may also cause cavitation within the fluid in the wellboreor in the proppant pack. The cavitation and acoustic streaming caused bythe piezoelectric crystal vibration and flow creates drag forces. Thedrag forces detach the attached fines within fractures in the reservoirand in the proppant pack. In this manner, the piezoelectric crystalsrestore fracture conductivity. Fluid flow from the wellbore subsequentlyflushes the fines out of the fractures and the proppant pack. In oneembodiment, the fractures can be cleaned from a distal end of eachfracture to a portion of each fracture proximate to the wellbore. Inanother embodiment, the features may be cleaned substantially along theentire fracture length and width into which the piezoelectric crystalsare pumped along with the proppant.

Still another aspect of the present invention is a method of enhancingthe production of a hydrocarbon reservoir. The method includes, but isnot limited to: (1) providing a wellbore extending a predeterminedlength and depth in the hydrocarbon reservoir; (2) providing a fluidwhich includes a proppant material and a plurality of piezoelectriccrystals; and (3) pumping the fluid into the wellbore. At least one ofthe plurality of piezoelectric crystals subsequently contracts inresponse to conditions within the wellbore. In one embodiment, at leastone of the plurality of piezoelectric crystals is transported into afracture in the hydrocarbon reservoir. In one embodiment, the fluid is ahydraulic fracturing fluid. In another embodiment, the fluid includeswater.

In one embodiment, the piezoelectric crystals are operable to contractin response to a reduction of at least one of a fluid temperature, afluid pressure, and a rate of fluid flow. In another embodiment, In oneembodiment, the piezoelectric crystals are operable to expand inresponse to an increase of at least one of a fluid temperature, a fluidpressure, and a rate of fluid flow.

The method may further include selecting a plurality of piezoelectriccrystals based on the frequencies the piezoelectric crystals willgenerate when activated. In one embodiment, the plurality ofpiezoelectric crystals will generate frequencies of betweenapproximately 0.5 kHz and approximately 1 GHz when activated. In anotherembodiment, the piezoelectric crystals will generate frequencies ofbetween approximately 1 kHz and 100 MHz when activated. In oneembodiment, the plurality of piezoelectric crystals may comprise two ormore different materials which exhibit piezoelectricity. Some of thepiezoelectric crystals may comprise a ceramic.

In one embodiment, the piezoelectric crystals comprise up toapproximately 50% by volume of the fluid. In another embodiment, thepiezoelectric crystals comprise up to approximately 25% by volume of thefluid. In still another embodiment, the piezoelectric crystals comprisebetween about 3% and about 50% by volume of the fluid.

In one embodiment, the piezoelectric crystals and the proppant materialare mixed in a one to one ratio (1:1) in the fluid. Alternatively, theratio of piezoelectric crystals to the proppant is at least one to five(1:5). In another embodiment, the piezoelectric crystals and proppantare mixed in a ratio of not more than five to one (5:1). Accordingly, inone embodiment, the ratio of piezoelectric crystals to the proppant isbetween approximately 1:5 and approximately 5:1 in the fluid. In anotherembodiment, the ratio of piezoelectric crystals to the proppant isbetween approximately 1:1.5 and approximately 1.5:1 in the fluid. Otherratios are contemplated.

Another aspect of the present invention is a fluid for transporting aproppant material and a plurality of piezoelectric crystals intofractures in a hydrocarbon reservoir. The fluid generally includes, butis not limited to: (1) a proppant material; (2) a plurality ofpiezoelectric crystals; and (3) a liquid selected to transport theproppant material and the plurality of piezoelectric crystals. The fluidcan be pumped into a wellbore such that the proppant material and atleast one of the piezoelectric crystals are transported into a fracturein the hydrocarbon reservoir. The piezoelectric crystals are operable toexpand and/or contract in response to changes in one or more of a fluidtemperature, a fluid pressure, and a rate of fluid flow in the wellbore.In one embodiment, the fluid is a hydraulic fracturing fluid. In anotherembodiment, the liquid includes water.

In one embodiment, the plurality of piezoelectric crystals are operableto generate frequencies of between approximately 0.1 kHz andapproximately 1 GHz when activated. In another embodiment, thepiezoelectric crystals are operable to generate frequencies of betweenapproximately 1 kHz and 100 MHz when activated. Optionally, thepiezoelectric crystals may have a variety of sizes and shapes.

In one embodiment, the piezoelectric crystals comprise up toapproximately 50% by volume of the fluid. In another embodiment, thepiezoelectric crystals comprise up to approximately 25% by volume of thefluid. In still another embodiment, the piezoelectric crystals comprisebetween about 3% and about 50% by volume of the fluid.

In one embodiment, the piezoelectric crystals and the proppant materialare mixed in a one to one ratio (1:1) in the fluid. Alternatively, theratio of piezoelectric crystals to the proppant is at least one to five(1:5). In another embodiment, the piezoelectric crystals and proppantare mixed in a ratio of not more than five to one (5:1). Accordingly, inone embodiment, the ratio of piezoelectric crystals to the proppant isbetween approximately 1:5 and approximately 5:1 in the fluid. In anotherembodiment, the ratio of piezoelectric crystals to the proppant isbetween approximately 1:1.5 and approximately 1.5:1 in the fluid. Otherratios are contemplated.

One aspect of the present invention is to provide a novel system andmethod for placement of a proppant pack in fractures of a wellbore. Inone embodiment, the system and method can be used to remove or decreaseproduction related damage to the proppant pack.

It is another aspect of the present invention to provide a system andmethod of placing proppants into fractures during a hydraulic fracturingoperation. In one embodiment, piezoelectric crystals are pumped into awellbore with proppants. In another embodiment, a downhole assembly ispositioned in the wellbore. The downhole assembly includes a screen or abar to which the piezoelectric crystals are mounted. In bothembodiments, the piezoelectric crystals expand or contract. Theexpansion and contraction is transferred to particles in the proppantpack and causes the proppant to move further into fractures within theformation.

Another aspect of the present invention is a system and method ofgenerating a variety of frequencies with piezoelectric crystals to cleanfactures in a formation. The frequencies are selected based oncharacteristics of the formation and the dimensions and geometry offractures in the formation. For example, the frequencies of thepiezoelectric crystals are selected based on the fracture length andwidth, formation type and mineralogy, gas or oil production, and thein-situ stress characteristics. The frequencies may be from low (kHz) toultrasonic (MHz) frequencies. The fractures may be naturally occurringor induced, such as by hydraulic fracturing operations. The system andmethod may be used in vertical, inclined, or horizontal wells which havefracture lengths of tens to hundreds of feet at multiple stages andextending along thousands of feet of the wellbore.

In one embodiment, the system and method includes a downhole assemblyincluding a plurality of piezoelectric crystals. The system and methodof embodiments of the present invention can be implemented in ahorizontal, a deviated, or a vertical well along the entirety of theproduction interval. In one embodiment, the system and method includecoupling multiple frequency acoustic waves and piezoelectric crystalspumped simultaneously with proppants into the well. Conditions in thewell and formation induce mechanical expansion and contraction of thepiezoelectric crystals. The expansion and contraction results indifferent displacements depending on the frequencies of eachpiezoelectric crystal. In this manner, the cleaning effects generated bythe system and method of the present invention can propagate throughoutthe fracture from the smallest fracture tip to the connection of thesefractures into the wellbore as well as into the natural fracture networkconnected through the hydraulic fracturing treatment.

The system and method of the present invention can be used to clean amajority of the length of a fracture. In one embodiment, a fracture canbe cleaned using the system and method of the present invention from abeginning of a fracture proximate to the wellbore to a tip of thefracture distal to the wellbore. Therefore, the cleaning effects arerealized for the full length, or a substantial portion of the length, offractures of horizontal, deviated or vertical wells and in the proppantpacked fractures covering the production interval. This cleaning helpsto maintain the production interval free of damage for the lifecycle ofthe well.

One aspect of the present invention is to provide a downhole assemblyfor positioning proppant in fractures along a production interval of awellbore to enhance flow rates from the wellbore. The assembly includes,but is not limited to: (1) a body configured to be positioned within theproduction interval of the wellbore; and (2) a plurality devicesinterconnected to the body. The devices are operable to expand andcontract. Activation of the devices creates displacement and/orcavitation in a fluid within the wellbore. In one embodiment, thedevices are piezoelectric crystals. In one embodiment, the piezoelectriccrystals have various sizes and frequencies to create the displacementnecessary to move proppants of various sizes. Optionally, thepiezoelectric crystals are selected based on specific reservoir andfracture characteristics. In another embodiment, the piezoelectriccrystals are spaced at predetermined intervals along the device body. Inone embodiment, additional piezoelectric crystals are mixed with theproppant and can be pumped together with the proppant into thefractures.

Optionally, the piezoelectric crystals are selected to generatedifferent frequencies when activated. In one embodiment, a first subsetof the piezoelectric crystals generate low frequencies. Optionally, thelow frequencies may be between approximately 0.1 kHz and approximately100 kHz when activated. In another embodiment, the first subset of thepiezoelectric crystals generate frequencies of between approximately 1kHz and approximately 10 kHz when activated.

In another embodiment, a second subset of the piezoelectric crystalsgenerate high frequencies. In one embodiment, the high frequencies arebetween approximately 10 kHz and approximately 1 GHz when activated.Optionally, the second subset of the piezoelectric crystals generatefrequencies are between approximately 10 kHz and approximately 10 MHzwhen activated. In another embodiment, the first subset of thepiezoelectric crystals are positioned on an exterior surface of thebody. In one embodiment, the second subset of the piezoelectric crystalsare positioned on an interior surface of the body. Additionally, oralternatively, the first subset of the piezoelectric crystals may bemixed with the proppant. Similarly, in one embodiment, the second subsetof the piezoelectric crystals is mixed with the proppant. In oneembodiment, the first subset of the piezoelectric crystals comprise afirst material that exhibits piezoelectricity. Optionally, the secondsubset of the piezoelectric crystals comprise a second material thatexhibits piezoelectricity.

In one embodiment, the body comprises a mesh material. Alternatively,the body comprises a solid material. In another embodiment, the body isgenerally cylindrical. In still another embodiment, the body includes asubstantially hollow bore with an interior surface. The piezoelectriccrystals can be positioned on at least one of an exterior surface of thebody and the interior surface within the hollow bore.

In one embodiment, the downhole assembly is triggered by contraction orelongation in the piezoelectric crystals. The contraction or elongationof the piezoelectric crystals results in displacement of the neighboringproppants. Associated displacements are carried to the neighboringproppants and piezoelectric crystals. In this manner, contraction andelongation of one piezoelectric crystal spreads to other piezoelectriccrystals and creates a chain reaction and continues through the proppantpack. The displacement motion is carried further through the proppantall the way into the tip of the hydraulic fractures.

In one embodiment, no downhole battery or surface power is needed toactivate the piezoelectric crystals. Optionally, one or more of thepiezoelectric crystals can be selectively activated. For example, in oneembodiment, the piezoelectric crystals may be activated if a pressuredifferential is not anticipated for a period of time.

Another aspect of the present invention is a system for improvingproduction of a fluid from a reservoir. In one embodiment, the systemincludes a mixture of proppant and piezoelectric crystals which arepumped into the well. Additionally, or alternatively, the system canoptionally include a downhole tool including piezoelectric crystals. Thepiezoelectric crystals of the system may be of multiple frequencies.Depending on the selected piezoelectric crystal frequency, a change inthe pressure, the flow rate and/or the temperature of the well willresult in activation of the of the piezoelectric crystals and causeexpansion and contraction of the piezoelectric crystals.

In one embodiment, the downhole tool is interconnected to an electricalsource. The electrical source may be a downhole battery. Additionally,or alternatively, the electrical source may be a wire-line to thesurface. The piezoelectric crystals can be interconnected to theelectrical source. In this manner, electricity can selectively beprovided to one or more of the piezoelectric crystals to activate thepiezoelectric crystals.

In one embodiment, activation of the apparatus creates displacementand/or movement of particles proximate to the piezoelectric crystals asa result of the piezoelectric crystal characteristics. The displacementand movement causes displacement in the neighboring proppants and otherpiezoelectric crystals. In this manner, a chain reaction of displacementwithin the proppant is created. The systems and methods of the presentinvention may be used to continuously maintain fracture conductivity andassociated flow without any decay in production volume throughout thefractures. Further, production from the well may continue while thepiezoelectric crystals in the wellbore and fractures expand and contractto clean the well of particles. In one embodiment, the particles are oneor more of proppant, fines, and completion residue. The fines may beman-made or naturally occurring.

One aspect of the present invention is a downhole assembly for enhancingflow rates from a wellbore. The downhole assembly comprises: (1) a bodyfor positioning within a production interval of the wellbore; and (2) aplurality of piezoelectric crystals interconnected to the body.Optionally, the piezoelectric crystals are interconnected to an exteriorsurface of the body. When the downhole assembly is positioned within thewellbore, at least one of the plurality of piezoelectric crystalsexpands or contracts in response to at least one of a change intemperature, a change in pressure, and a change in fluid flow rate inthe wellbore.

In one embodiment, the piezoelectric crystals have predetermined sizes.In another embodiment, the plurality of piezoelectric crystals havesizes and frequencies selected based on characteristics of the wellboreincluding at least one of the depth, length, temperature, flow rate,hydraulic fracturing interval, reservoir permeability, formation type,and reservoir porosity.

In one embodiment, the body includes a bore defining an interiorsurface. Optionally, at least one piezoelectric crystal isinterconnected to the interior surface of the body.

In another embodiment, a first subset of the plurality of piezoelectriccrystals are selected to generate low frequencies when activated.Optionally, the low frequencies are between approximately 0.1 kHz toapproximately 100 kHz when activated. Alternatively, the low frequenciesmay be between approximately 1 kHz to approximately 10 kHz whenactivated. In one embodiment, the first subset of the plurality ofpiezoelectric crystals are positioned on the exterior surface of thebody.

Additionally, or alternatively, the downhole assembly may optionallyinclude a second subset of the plurality of piezoelectric crystals whichare selected to generate high frequencies when activated. In oneembodiment, the high frequencies are between approximately 10 kHz andapproximately 1 GHz when activated. In another embodiment, the highfrequencies are between approximately 10 kHz and approximately 100 MHzwhen activated. Additionally, or alternatively, the second subset of theplurality of piezoelectric crystals may be positioned on an interiorsurface of the body.

In one embodiment, the body comprises a solid bar. Alternatively, thebody comprises a mesh material. In one embodiment, the first subset ofthe piezoelectric crystals comprise a first material that exhibitspiezoelectricity. Optionally, the second subset of the piezoelectriccrystals comprise a second material that exhibits piezoelectricity.

The downhole assembly may further comprise a power source to provideelectricity to the plurality of piezoelectric crystals. Optionally, thedownhole assembly includes a controller operable to send a signal toactivate and deactivate the plurality of piezoelectric crystals.

Another aspect is a method of enhancing a flow rate from a wellbore in areservoir, comprising: (1) positioning a downhole assembly in aproduction interval of the wellbore, the downhole assembly generallyincluding (i) a body; and (ii) piezoelectric crystals interconnected tothe body; and (2) triggering at least one of the plurality ofpiezoelectric crystals. When triggered, the at least one piezoelectriccrystal expands and/or contracts which causes fines in fractures of thereservoir move to repair and improve the permeability of a hydraulicreservoir proximate to the wellbore.

The method may further comprise flowing fluid from the wellbore. In thismanner, the fines are flushed out of the fractures. In one embodiment,the at least one piezoelectric crystal is triggered by a change in acondition within the well bore. More specifically, in one embodiment,the at least one piezoelectric crystal is triggered by a change in oneor more of a temperature, a pressure, and a rate of fluid flow in thewellbore.

Optionally, the body comprises a screen or a solid bar for placement inthe wellbore. In one embodiment, a first subset of the piezoelectriccrystals are operable to generate low frequencies. In one embodiment,the low frequencies generated by the piezoelectric crystals are betweenapproximately 0.1 kHz to approximately 100 kHz. The downhole assemblymay optionally include a second subset of the piezoelectric crystalswhich are operable to generate high frequencies. In one embodiment, thehigh frequencies generated by the piezoelectric crystals are betweenapproximately 10 kHz to approximately 100 MHz.

The method may optionally include selecting at least one of a pattern, asize, and a frequency of the plurality of piezoelectric crystals. Thepattern, size, and/or frequency of the piezoelectric crystals may bebased on a characteristic of the wellbore and the reservoir. The methodmay further comprise selecting piezoelectric crystals of two or moredifferent materials which exhibit piezoelectricity.

Another aspect of the present invention is a method for implementing a4-D real-time stimulation application. The method comprises: (1)measuring various wellbore and reservoir conditions; (2) determining thechanges in production and production related factors from theseconditions; and (3) activating a piezoelectric crystal to clean damagefrom the wellbore and surrounding reservoir to improve production. Inone embodiment, the piezoelectric crystal is positioned within afracture of the reservoir. Optionally, the piezoelectric crystal ispositioned within the wellbore. In another embodiment, the piezoelectriccrystal is affixed to a downhole assembly positioned in the wellbore.

In one embodiment, a plurality of piezoelectric crystals are positionedin the wellbore. Individual piezoelectric crystals of the plurality ofpiezoelectric crystals may react differently to changes of pressure,temperature, and flow rate within the wellbore. For example, twopiezoelectric crystals may react differently based on differences intheir relative size, position within the wellbore or fractures, anddifferences in their materials. In one embodiment, a first one of theplurality of piezoelectric crystals generates a first frequency whenactivated. A second one of the plurality of piezoelectric crystalsgenerates a second frequency when activated. By measuring frequenciesgenerated by the piezoelectric crystals, changes in conditions withinthe wellbore and the fractures in the geologic formation can bedetermined and located.

Another aspect of the present invention is a method of utilizingacoustic waves in a wellbore. The acoustic waves create acoustic induceddrag forces to transport proppants from the wellbore into fractures inthe production interval of a well to encourage proper filling of thefracture with proppant. In this manner, the systems and methods of thepresent invention improve the proppant pack stability and maintenance ofthe fracture length, width and height throughout the lifecycleproduction. One method of creating these acoustic waves is by the use ofpiezoelectric crystals. The piezoelectric crystals may be mixed with theproppant and pumped into the well. Additionally, or alternatively,piezoelectric crystals can be attached to a downhole assembly positionedwithin the well. Optionally, the downhole assembly includes a screen. Inone embodiment, piezoelectric crystals with different frequencies areinterconnected to one or more of an inside surface and an outsidesurface of the screen.

Although generally referred to herein as piezoelectric “crystals,” itshould be appreciated that the current invention may be used with anymaterial which exhibits piezoelectricity. Accordingly, the term“piezoelectric crystals” as used herein refers to any type of materialwhich exhibits piezoelectricity. The material may be natural orman-made. The material may be a crystal. Alternatively, the material maybe a ceramic. In one embodiment, the piezoelectric crystals are selectedto have a hardness that is greater than the proppants.

As used herein, the term “fracture” means a fracture in a reservoir ofany type. The fracture may be hydraulically induced (or “man-made”) oran open, natural fracture.

Unless otherwise indicated, all numbers expressing quantities,dimensions, conditions, ratios, ranges, and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about” or “approximately”. Accordingly, unlessotherwise indicated, all numbers expressing quantities, dimensions,conditions, ratios, ranges, and so forth used in the specification andclaims may be increased or decreased by approximately 5% to achievesatisfactory results.

The term “a” or “an” entity, as used herein, refers to one or more ofthat entity. As such, the terms “a” (or “an”), “one or more” and “atleast one” can be used interchangeably herein.

The use of “including,” “comprising,” or “having” and variations thereofherein is meant to encompass the items listed thereafter and equivalentsthereof as well as additional items. Accordingly, the terms “including,”“comprising,” or “having” and variations thereof can be usedinterchangeably herein.

It shall be understood that the term “means” as used herein shall begiven its broadest possible interpretation in accordance with 35 U.S.C.,Section 112(f). Accordingly, a claim incorporating the term “means”shall cover all structures, materials, or acts set forth herein, and allof the equivalents thereof. Further, the structures, materials, or actsand the equivalents thereof shall include all those described in theSummary of the Invention, Brief Description of the Drawings, DetailedDescription, Abstract, and Claims themselves.

The Summary of the Invention is neither intended, nor should it beconstrued, as being representative of the full extent and scope of thepresent invention. Moreover, references made herein to “the presentinvention” or aspects thereof should be understood to mean certainembodiments of the present invention and should not necessarily beconstrued as limiting all embodiments to a particular description. Thepresent invention is set forth in various levels of detail in theSummary of the Invention as well as in the attached drawings and theDetailed Description and no limitation as to the scope of the presentinvention is intended by either the inclusion or non-inclusion ofelements or components. Additional aspects of the present invention willbecome more readily apparent from the Detailed Description, particularlywhen taken together with the drawings.

The accompanying drawings, which are incorporated herein and constitutea part of the specification, illustrate embodiments of the invention andtogether with the Summary of the Invention given above and the DetailedDescription given below serve to explain the principles of theseembodiments. In certain instances, details that are not necessary for anunderstanding of the disclosure or that render other details difficultto perceive may have been omitted. It should be understood, of course,that the present invention is not necessarily limited to the particularembodiments illustrated herein. As will be appreciated, otherembodiments are possible using, alone or in combination, one or more ofthe features set forth above or described below. For example, it iscontemplated that various features and devices shown and/or describedwith respect to one embodiment may be combined with or substituted forfeatures or devices of other embodiments regardless of whether or notsuch a combination or substitution is specifically shown or describedherein. Additionally, it should be understood that the drawings are notnecessarily to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a well formed in a geologic formation;

FIG. 2A is an expanded schematic diagram of a downhole assembly of thepresent invention positioned within a horizontal segment of the wellboreof FIG. 1;

FIG. 2B is a cross-sectional view of the downhole assembly of FIG. 2Ataken along line 2B-2B;

FIG. 3 is a schematic diagram of another embodiment of a downholeassembly of the present invention;

FIG. 4 is a cross-sectional view of the downhole assembly of FIG. 3taken along line 4-4.

FIG. 5 is a schematic diagram showing piezoelectric crystals pumpedtogether with proppants into a wellbore according to an embodiment ofthe present invention;

FIG. 6 is a side elevation view of a fracture into which proppants andpiezoelectric crystals have been pumped;

FIG. 7 is an illustration of a mixture of proppant and piezoelectriccrystals positioned in a section of casing and fractures extending fromthe casing into a geologic formation;

FIG. 8 is a schematic diagram similar to FIG. 2A and illustrating adownhole assembly positioned in a generally vertical portion of awellbore and including piezoelectric crystals mixed with proppant in thewellbore and fractures of a geologic formation;

FIG. 9 illustrates forces acting on particles; and

FIGS. 10-12 are graphs of the detachment ratio of particles as a ratioof particles size for various frequencies.

A list of the various components shown in the drawings and associatednumbering is provided herein:

Number Component 1 Well 2 Vertical segment of well 3 Horizontal segmentof well 4 Geologic formation 6 Fractures 8 Tip of fractures 10 Casing 12Casing aperture 14 Proppant 16 Downhole assembly 18 Body 20 Longitudinalaxis 22 Exterior surface 24 Hollow bore 26 Interior surface 28Piezoelectric crystals 30 Power source (optional) 32 Control system(optional)

DETAILED DESCRIPTION

Referring now to FIG. 1, a schematic view of a well 1 formed in ageologic formation 4 is generally illustrated. The well 1 may include asegment 2 that is generally vertical and a segment 3 that is generallyhorizontal. The segments 2, 3 may be sloped or inclined at variousangles. As will be appreciated by one of skill in the art, the well 1can have a variety of orientations and any number of segments 2, 3.

A downhole assembly 16 is illustrated positioned in the generallyhorizontal segment 3. Although not illustrated, the downhole assembly 16may also be positioned in the generally vertical segment 2. Optionally,one or more of a power source 30 and a control system 32 can be operablyconnected to the downhole assembly 16.

Referring now to FIGS. 2A-2B, an expanded view of a portion of the well1 is illustrated with one embodiment of a downhole assembly 16 of thepresent invention generally illustrated within a horizontal segment 3 ofthe well. The downhole assembly 16 includes piezoelectric crystals 28and is positioned in a horizontal well 1 while proppant 14 is injected.The downhole assembly 16 is configured to be positioned in a casing 10positioned in a wellbore which extends into a geologic formation 4. Thecasing 10 includes a plurality of apertures 12, such as perforations orslots, which allow access to fractures 6 which extend into the reservoirformation 4. The wellbore casing 10 and fractures 6 are illustratedafter being packed with a proppant 14. The fractures 6 may extendhundreds or thousands of feet from the casing 10.

The downhole assembly 16 generally includes a body 18. In oneembodiment, the body 18 comprises a screen. The screen 18 can be apermanent screen for proppant pack placement and stimulation.Alternatively, the screen can be retrievable for use in anotherwellbore.

In one embodiment of the downhole assembly 16, a plurality ofpiezoelectric crystals 28 are attached to the body 18. Specifically, inone embodiment, the piezoelectric crystals 28 can be interconnected toan exterior surface 22 of the body. The piezoelectric crystals 28 areoperable to create displacement and/or cavitation and acoustic streamingin a fluid when activated.

Optionally, the body 18 has a cylindrical shape extending along alongitudinal axis 20. In one embodiment, the downhole assembly body 18is comprised of a mesh material similar to screens used in the wells toprevent sand production.

In one embodiment, the downhole assembly body 18 includes a hollow bore24. The hollow bore includes an interior surface 26. A plurality ofpiezoelectric crystals 28 can optionally be interconnected to theinterior surface 26.

The piezoelectric crystals 28 can optionally be substantially evenlyspaced on one or more of the exterior and interior surfaces 22, 26 ofthe body 18. In another embodiment, the piezoelectric crystals 28 canhave an uneven or random spacing. The spacing between the piezoelectriccrystals 28 can be adjusted based on the type of reservoir, reservoirmineralogy and level of damage anticipated. Optionally, a group ofpiezoelectric crystals can be concentrated at a first portion of thebody 18. A second portion of the body 18 may have fewer, or no,piezoelectric crystals 28 depending on the type of the reservoir and thedamage to be removed. In this manner, the amount or location ofcavitation generated by the downhole assembly 16 can be altered alongthe length or the diameter of the body 18.

Optionally, in one embodiment, the piezoelectric crystals 28 are ofsubstantially the same size and shape. Alternatively, one or more of thepiezoelectric crystals 28 may have a different size or a different shapethan others of the piezoelectric crystals. Accordingly, thepiezoelectric crystals 28 may or may not be the same size. By selectingpiezoelectric crystals 28 of various sizes, the downhole assembly 16 cancreate a variety of wavelengths that will help detach fines and debrisof different sizes from the proppant 14 and fractures 6. Thepiezoelectric crystals on the body 18 can be placed based on the desiredcleanliness of the specific locations of the well.

In one embodiment, various sizes of piezoelectric crystals 28 can beinterconnected to the downhole assembly 16 depending on the application.Optionally, the size of the piezoelectric crystals 28 is selected basedon one or more of: (i) characteristics of the fractures 6, such as thelength, width and height of the fractures; (ii) the density anddistribution of the natural fractures; (iii) characteristics of theformation 4 including the compaction properties and strength of theformation; (iv) the petrophysical properties such porosity,permeability, grain size, pore throat size, formation mineralogy andtexture; (v) the composition and characteristics of the reservoir fluid;and (vi) the type and petrophysical characteristics of the proppant packincluding the proppant size used for packing, porosity, permeability ofthe proppant pack, and the fracturing fluid composition andcharacteristics.

In one embodiment, the piezoelectric crystals may be one of material.Alternatively, in another embodiment, downhole assembly 16 may include aplurality of piezoelectric crystals formed of two or more differentmaterials which exhibit piezoelectricity. For example, the downholeassembly 16 may include a first plurality of piezoelectric crystalsformed of a first piezoelectric material and a second plurality ofpiezoelectric crystals formed of a second piezoelectric material. In oneembodiment, the first piezoelectric material may have a first hardnessand the second piezoelectric material can have a second hardness.

Once the piezoelectric crystals 28 are interconnected to the downholeassembly, the downhole assembly 16 is positioned in the well 1 insections where the proppant pack will be established. The downholeassembly 16 will create localized vacuums by cavitation during theproppant injection during the fracturing process. The process can becontinued throughout the lifecycle of the well as needed. Utilizing thedownhole assembly 16 with piezoelectric crystals 28 during the fracturestimulation process will aid in efficient proppant placement for aneffective proppant pack from the tip 8 of the fractures 6 to the insideof the casing along the horizontal, inclined or vertical well.

The piezoelectric crystals 28 are triggered during the fracturing totransport the proppant deeper into the reservoir 4 into the tip 8 of thefractures. The piezoelectric crystals 28 trigger automatically based onthe piezoelectric crystal properties and the change in pressure,temperature and flow rate within the well. More specifically,piezoelectric contraction and elongation as a result of pressure,temperature and/or flow rate changes cause one piezoelectric crystal 28to deform neighboring proppants 14 and also other piezoelectric crystals28. Accordingly, one piezoelectric crystal 28 can transfer electricityto other piezoelectric crystals 28 which activates the otherpiezoelectric crystals 28 for the continuous stimulation of the proppantpack.

When triggered, the piezoelectric crystals 28 expand and contract whichhelps efficiently place the proppants in the fractures. After theproppant is placed in the fractures, expansion and contraction of thepiezoelectric crystals 28 will help maintain the conductivity andpermeability of the fractures as close to initial levels when productionstarted. The piezoelectric crystals 28 will continue whenever changes inone or more of pressure, temperature, and flow rates take place duringthe lifecycle of the well. When a stable flow rate and pressure areachieved, production from the well will continue until a change inpressure, temperature, or flow rate occurs due to damage of the proppantpack or reservoir. The change will trigger one or more of thepiezoelectric crystals 28, causing elongation and/or contraction thatstimulate the proppant pack. The stimulation prevents any blockage anddamage in the proppant pack and/or formation resulting continuouslyself-stimulating reservoir to maintain the permeability as close to theinitial permeability when the production begins. Fines are dislodgedfrom the proppant pack and then removed from the well by fluid flow.

In one embodiment, the piezoelectric crystals 28 automatically activatein response to changes in conditions within the well. More specifically,changes in one or more of pressure, temperature, and flow rate can causea piezoelectric crystal 28 to expand or contract automatically. Thedisplacement and cavitation caused by pressure and temperature changeswithin the well cause the piezoelectric crystal 28 to expand and/orcontract. The piezoelectric crystals 28 are self-powered a result of thecreated electric charge from the contraction or elongation. Accordingly,in one embodiment, no external source power is needed by the downholeassembly 16.

The expansion and contraction of one piezoelectric crystal 28 causesdisplacement of nearby particles, include proppant, other piezoelectriccrystals 28, and fines within the well. The properties of thepiezoelectric crystals allow the displacement of nearby piezoelectriccrystals. By selecting multiple frequency piezoelectric crystals for thedownhole assembly 16, different amounts of displacements can be createdat different locations within the casing 10.

As the space is limited downhole, once one piezoelectric crystal 28expands or contracts, then the displacement is transferred into theneighboring proppants, particles, and piezoelectric crystals 28. Thisdisplacement activates other piezoelectric crystals 28 in a chainreaction. Hence, the piezoelectric crystals 28 of the downhole assembly16 may be continuously activated to clean the well for a long period oftime or momentarily depending on the pressure, flow rate and/or thermalstability of the well.

The piezoelectric crystals 28 can be used to produce an acoustic forceby a process that coverts electrical energy to mechanical energy andvisa-versa. When piezoelectric crystalline matter is subjected to amechanical force, the crystal becomes electrically polarized. Appliedcompressional and tensional forces on the crystalline matter generatesvoltages of opposite polarity, and in proportion to the applied force.Conversely, when piezo crystalline matter is exposed to an electricfield, it is elongated or shortened according to the polarity of thefield, and in proportion to the strength of the field. Therefore,piezoelectric crystals 28 mounted on the downhole assembly 16 in thewellbore create localized vacuums along the downhole assembly 16 by thepiezoelectric crystal caused displacement (contraction and elongation)and in some cases cavitation when the piezoelectric crystal 28 elongatesand shortens due to the electrical charge.

Cavitation usually occurs when a liquid is subjected to rapid changes ofpressure. The pressure changes cause the formation of cavities in theliquid where the pressure is relatively low, such as during productionof the well. When subjected to higher pressure, the voids implode andcan generate a shock wave. Cavitation inception occurs when the localpressure falls sufficiently far below the saturated vapor pressure, avalue given by the tensile strength of the liquid at a certaintemperature. This displacement and/or cavitation creates additional dragforces which help prevent screen out of the proppant-fluid mixture, yetforce the proppant-fluid mixture to propagate toward the tip 8 (ordistal ends) of fractures 6, toward the space between the wellbore andthe screen and all other vertical, deviated and horizontal fracturespresent in the reservoir. Cavitation is one part of the cleaningmechanism that may take place at a specific pressure and temperaturewith a specific fluid. The real cleaning is conducted through theparticle detachment and carriage of the removed particles with the fluidin the fractures out of the fracture 6 into the wellbore and to thesurface.

Spacing of the piezoelectric crystals 28 is important for thepiezoelectric crystals 28 to effectively transfer their elongation andcontraction to the nearest proppants for continuation of the movementand related stimulation and to accomplish the maintenance ofpermeability of the well and geologic formation. Accordingly, thelocation and spacing of the piezoelectric crystals 28 may be selectedbased on conditions in the wellbore.

Optionally, external power can be supplied to activate the piezoelectriccrystals 28. Accordingly, in one embodiment, the downhole assembly 16 isinterconnected to a power source 30. One or more of the piezoelectriccrystals 28 may be interconnected to the power source. The power source30 may comprise a battery positioned within the wellbore. Optionally,the battery may be associated with the downhole assembly 16.Additionally, or alternatively, the power source 30 may comprise a fiberoptic line. In another embodiment, the power source 30 is a wireline tothe surface.

In one embodiment, the piezoelectric crystals can be individuallyactivated. In another embodiment, two or more of the piezoelectriccrystals can be activated substantially simultaneously.

The piezoelectric crystals can also be activated in a pattern. Forexample, a first group of piezoelectric crystals 28 can be periodicallyactivated. A second group of piezoelectric crystals can also beperiodically activated. The periods of activation of the first andsecond groups can be the same or different. In one embodiment,piezoelectric crystals 28A positioned on the exterior surface of thedownhole assembly 16 may be activated separately from piezoelectriccrystals 28B positioned on the interior surface 26.

Optionally, the piezoelectric crystals 28 can be divided into any numberof groups. In one embodiment, a piezoelectric crystal can be in morethan one group. For example, a first group may comprise the odd numberedpiezoelectric crystals along the length of the body 18. A second groupcan comprise the even numbered piezoelectric crystals. A third group mayinclude every fifth piezoelectric crystal. Accordingly, the firstpiezoelectric crystal can be in the first and third groups. A fourthgroup may comprise piezoelectric crystals positioned on an exteriorsurface of the body and a fifth group can include piezoelectric crystalspositioned within the hollow bore 24 of the body 18. In one embodiment,each group of piezoelectric crystals 28 can be independently activated.In this manner, the downhole assembly 16 may have five or more modes ofoperation, each mode creating different patterns, intensities, orlocations of cavitation in fluid around the downhole assembly.

Piezoelectric crystals can also be grouped according to their positionrelative to a longitudinal axis 20 of the body 18. For example, a sixthgroup can include the piezoelectric crystals positioned above thelongitudinal axis 20 as illustrated in FIG. 2. A seventh group caninclude the piezoelectric crystals positioned to the right of thelongitudinal axis 20 when viewed in FIG. 2. Other groups ofpiezoelectric crystals are contemplated.

In one embodiment, a control system 32 is operable to activate one ormore of the piezoelectric crystals. Optionally, the control system 32 isinterconnected to the power source 30. More specifically, the controlsystem 32 in one embodiment is configured to selectively supply power toone or more of the piezoelectric crystals 28 of the downhole assembly16.

In one embodiment, the control system 32 of the present invention canassign the piezoelectric crystals 28 to one or more groups. In oneembodiment, the control system 32 can assign the piezoelectric crystals28 to the one or more groups based on one or more of a geometry of thefractures 6 and the formation 4 type.

In one embodiment, the piezoelectric crystals 28 are operable togenerate predetermined frequencies when activated. The frequenciesgenerated by the piezoelectric crystals 28 may be selected based oncharacteristics of one or more of the fractures 6 and the formation 4.For example, the piezoelectric crystals 28 can be selected to generatepredetermined frequencies based on at least one of the length and thewidth of fractures in the geologic formation 4. Additionally, oralternatively, in another embodiment, one or more frequency generated bythe piezoelectric crystals is based on the formation type. Thefrequencies of the piezoelectric crystal 28 may also be selected basedon formation type and mineralogy, gas or oil production, and the in-situstress characteristics.

Optionally, all of the piezoelectric crystals 28 generate the samefrequency when activated. In one embodiment, at least some of thepiezoelectric crystals generate low frequencies. The low frequencies maybe in the low Hz to hundreds of Hz. Optionally, the low frequencypiezoelectric crystals generate frequencies of between about 0.1 kHz toless than approximately 1 MHz when activated. In one embodiment, the lowfrequency piezoelectric crystals 28A are positioned on an exteriorsurface 22 of the downhole assembly 16. The low frequency piezoelectriccrystals 28A generally aid in the cleaning of the proppants 14 withinthe fractures 6.

In another embodiment, at least some of the piezoelectric crystals 28generate high frequencies. The high frequencies may vary between low Hzto MHz or hundreds of MHz. Optionally, the high frequency piezoelectriccrystals generate frequencies of between about 10 kHz to less thanapproximately 1 GHz when activated. In another embodiment, thefrequencies generated by the high frequency piezoelectric crystals isbetween about 100 Hz to about 10 MHz. Optionally, the high frequencypiezoelectric crystals 28B are positioned within the hollow bore 24 ofthe body 18. In one embodiment, the high frequency piezoelectriccrystals generally aid in cleaning the proppant within the screen 18 ofthe body of the downhole assembly 16.

The piezoelectric crystals 28 are selected depending on which frequencywill work best for the specific formation and/or fracture network.Typically a range of frequencies will be beneficial since the fines andproppants causing damage or reducing the fracture conductivity aremulti-sized. Different size of fines can be detached or displaced withdifferent frequencies generated by vibrations of the piezoelectriccrystals 28. Further, the material of the fines and other particles tobe cleaned from the proppant 14 and the flow of the well may also beconsidered when selecting frequencies of the piezoelectric crystals 28.Methods of selecting piezoelectric crystals 28 of appropriatefrequencies are described hereinafter.

One method of using the downhole assembly 16 includes cleaning the well1 of drilling fluid, mud cake, and other drilling damage when the targetdistance for a vertical, deviated or horizontal drilling is reached.Then the downhole assembly 16 of the present invention is connected tothe drill pipe, and placed within the vertical, deviated or horizontalwell. In one embodiment, the diameter of the downhole assembly 16 isselected based on the target production from the well and the companyproduction management procedures. In one embodiment, the diameter of thedownhole assembly may be a diameter as large as the wellbore diameterwhen the downhole assembly 16 will be permanently placed in thewellbore. Alternatively, the diameter of the downhole assembly may besmaller than the wellbore diameter to allow for retrieval and reuse ofthe downhole assembly.

The downhole assembly 16 can be retrieved from the wellbore whencontinued production of the well is uneconomical. In this manner, thedownhole assembly can be reused in other wells to reduce costs once.Alternatively, the downhole assembly can be permanently positioned inthe wellbore. The permanent deployment of downhole assemblies 16 of thepresent invention is also economical as it provides a lifetime ofreliable fracture stimulation eliminating the need for refracturing orother stimulation procedures. More specifically, because thepiezoelectric crystals 28 will automatically expand and/or contract inresponse to conditions within the well, the downhole assembly 16 willcontinuously clean the proppant 14 without external power and withoutinput from a technician at the well. Accordingly, the downholeassemblies of the present invention provide a reliable system to cleanproppant and improve well productivity without external power oradditional labor costs.

Referring now to FIGS. 3-4, another embodiment of a downhole assembly16B of the present invention is generally illustrated. The downholeassembly 16B is similar to the downhole assembly 16A described inconjunction with FIGS. 2A-2B and includes many of the same, or similar,features. In addition, the downhole assembly 16B operates in a mannersimilar to the downhole assemble 16A. The downhole assembly 16B can bepositioned in a well segment with any orientation, including ahorizontal well segment 3 or a vertical well segment 2.

Notably, the downhole assembly 16B includes a body 18 that is solid.Piezoelectric crystals 28 are interconnected to an exterior surface 22of the body 18. In one embodiment, one or more of a density, a diameter,and a material of the body 18 are selected based on the geometry of thefractures 6 or the formation 4 through which the wellbore is formed.More specifically, one or more of the density, the diameter, and thematerial of the body 18 can be selected to adjust the displacementand/or cavitation generated by the downhole assembly 16B and thepiezoelectric crystals 28. In one embodiment, the body 18 comprisessteel casing. Although not illustrated, the body 18 may optionallyinclude a hollow bore the same as or similar to the hollow bore 24illustrated in conjunction with FIGS. 2A-2B.

The downhole assembly 16 can be used to help displace (or transport) theproppant 14 further into the fractures 6 by utilizing the larger dragforces created by the induced acoustic streaming and acousticcavitation. The downhole assembly 16 can be employed after the firstfluid pad has been implemented, and that coupled with the drag forcesintroduced by the piezoelectric crystals 28 mounted on the body 18results in the proppants 14 being displaced further into the fractures6.

Referring now to FIGS. 5-7, in one embodiment of the present invention,piezoelectric crystals 28C are mixed with proppant 14. The mixture ofpiezoelectric crystals 28C and proppant 14 can subsequently be pumpedinto the well 1. The mixture of piezoelectric crystals 28C and proppant14 can be pumped into the well during, or after, the fracturingoperation. In one embodiment, the piezoelectric crystals 28C are mixedwith the proppant in a fluid. In this manner, the piezoelectric crystals28C are transported into the wellbore and into the hydraulically createdand naturally existing connected fractures 6. In one embodiment, thefluid is a hydraulic fracturing fluid.

Similar to the piezoelectric crystals 28 used with the downholeassemblies 16 described herein, the piezoelectric crystals 28C may be ofequal or varying sizes. In one embodiment, the piezoelectric crystalsmay be one of material. Alternatively, in another embodiment, themixture of piezoelectric crystals and proppant may include a pluralityof piezoelectric crystals formed of two or more different materialswhich exhibit piezoelectricity. For example, the mixture may include afirst plurality of piezoelectric crystals formed of a firstpiezoelectric material and a second plurality of piezoelectric crystalsformed of a second piezoelectric material. In one embodiment, the firstpiezoelectric material may have a first hardness and the secondpiezoelectric material can have a second hardness. Optionally, the firstpiezoelectric material may react to changes in temperature, pressure,and flow rate within the wellbore differently than the secondpiezoelectric material.

As generally illustrated, the piezoelectric crystals 28C may betransported to the tips 8 of the fractures 6. More specifically, thepiezoelectric crystals 28C can be positioned in the fracture tips 8outside of the well bore 1 and casing 10.

The size and frequencies of the piezoelectric crystals 28C are selectedbased on the natural fractures present, the hydraulic fracturing designand characteristics of the formation to be fractured and fractures. Inone embodiment, the size and frequency of the piezoelectric crystals 28Cas well as the concentration of the piezoelectric crystals 28C in theproppant pack is determined by the fracture volume, reservoircharacteristics and in situ reservoir stress magnitudes to determine thelevel of increases and decreases in the stress and temperatures duringthe production.

The ratio of the piezoelectric crystals 28C to proppant 14 may also beselected based on the formation, well type, fracture characteristics,and other properties of the well and the proppant. In one embodiment,the piezoelectric crystals and proppant are mixed at approximately a 1:1ratio by volume. In one embodiment, piezoelectric crystals comprise upto about 80% of the volume of the mixture of piezoelectric crystals andproppant. In another embodiment, the mixture comprises at least about20% piezoelectric crystals by volume. In still another embodiment, themixture comprises between about 20% and about 80% piezoelectric crystalsand between about 80% and about 20% proppant by volume. In anotherembodiment, the mixture comprises between about 40% and about 60%piezoelectric crystals and between about 60% and about 40% proppant byvolume. In still another embodiment, piezoelectric crystals comprisebetween about 45% and about 55% of the mixture and the proppantcomprises between about 55% and about 45% of the mixture.

In one embodiment, the mixture comprises between about 0.20 wt. % andabout 0.80 wt. % piezoelectric crystals and between about 0.20 wt. % andabout 0.80 wt. % of a proppant material. In another embodiment, themixture comprises between about 0.40 wt. % and about 0.60 wt. %piezoelectric crystals and between about 0.40 wt. % and about 0.60 wt. %of the proppant material. In still another embodiment, the mixturecomprises between about 0.45 wt. % and about 0.55 wt. % piezoelectriccrystals and between about 0.45 wt. % and about 0.55 wt. % of theproppant material.

The mixture of proppant and piezoelectric crystals may subsequently beadded to a fluid, such as water or a hydraulic fracturing fluid. Thefluid including the mixture of proppant and piezoelectric crystals canthen be pumped into a wellbore.

Referring now to FIG. 8, in one embodiment of the present invention,piezoelectric crystals 28C can be mixed with the proppant 14 and pumpedinto the well 1. A downhole assembly 16A or 16B including additionalpiezoelectric crystals 28 may also be positioned within the well 1.Although the downhole assembly 16A and piezoelectric crystals 28C areillustrated in a generally vertical segment 2 of the well, as previouslydescribed, the downhole assembly 16A and piezoelectric crystals 28C maybe positioned in any segment 2, 3 of a well 1.

One method of the present invention includes real-time monitoring of thequality of the proppant pack. More specifically, the proppant pack ismonitored between the production interval of the wellbore and thedownhole assembly 16 with the piezoelectric crystals 28, along with theproppant pack from the interior of a bore of the downhole assembly tothe tip of the vertical/deviated/horizontal natural and inducedfracture. This monitoring method may utilize existing fracture mappingtechniques such as surface tiltmeter measurements, surface and/orin-well micro-seismic monitoring, or fiber optic monitoring from whichthe change in the dimensions of the fractures can be determined. Thefiber optic monitoring may include one or more of DistributedTemperature Sensing (DTS), distributed acoustic sensing (DAS),Distributed Pressure Sensing (DTP), and others. When continuousstimulation using piezoelectric crystals is used, then the smalldisplacements will be recorded providing information about any changetaking place in the fracture dimensions that fracture conductivity is afunction of.

The system and methods of the present invention are expected toeliminate the need for refracturing and reduce the associated water useand groundwater contamination risk. Thus, using a downhole assembly 16including piezoelectric crystals 28 and real-time monitoring of theproppant pack placement, as well as repeated use of the piezoelectriccrystal displacement and in some occasions acoustic cavitation inducedby the fluctuations in the pressure and temperature in various cornersof the proppant pack will result displacement of neighboringpiezoelectric crystals and proppants that will not allow settlement anddetachment of the fines or any debris received passing through with theproduced fluids. Implementing this method and apparatus will alsoenhance economically viable production with the savings from continuousmaintenance and refracturing needs and environmentally friendlyproduction from tight gas sands, shale gas, tight oil and otherconventional, deep-water and unconventional resource wells by allowingfor higher production rates and reducing the need for refracturingoperations.

The methods and apparatus of embodiments of the present invention canalso be utilized in deep-water poorly consolidated high permeabilityreservoir completions. For example, the downhole assemblies 16 can beused in deep-water poorly consolidated reservoirs with gravelcompletions for efficient gravel packing prior, during, and postfrac-pack operations throughout the lifecycle of deep-water andultra-deep-water wells. The methods and apparatus could be employedduring the frac-pack fracture stimulations of these reservoirs. Thismethod and apparatus would help ensure effective gravel packing andwould aid the removal of damage within the gravel pack due to theplugging of the pore throats and gas and water blocking throughsimultaneously pumping piezoelectric crystals of suitable size andfrequencies when frac-pack operation is conducted.

Referring now to FIGS. 10-12, the frequencies of the piezoelectriccrystals 28 may be selected based in part on characteristics ofparticles, such as fines, in the proppant or the formation. For thehydrodynamic problem for detachment of fines or particles from surfacesfor cleaning, there is no net lift force acting on the particle. Thereis a tangential force (drag force) that is 1.7 times greater than thedrag force created on a sphere in an unbounded medium. A torque is alsoexerted on the particle. The fluid velocity is evaluated at a distanceR, the radius of the particle to be removed, away from the plane wallfor calculating drag force:

F _(D)=1.7(6πμRu| _(R))

Here “μ” is viscosity of the fluid, and “u” is the flowing fluidvelocity. For a rolling mechanism, torque balance is evaluated as:

F _(D) *R=F _(adhesion) *a

can be used to calculate the critical hydrodynamic force at whichparticle detachment may occur. Here, F_(adhesion) is the Van der Waalsforce acting on the particle at equilibrium separation distance, and “a”is the contact radius for a particle calculated using the adhesion forceas the body force causing deformation. The adhesion force is calculatedconsidering the distance of separation at the contact zone as well as atthe noncontact zone. The calculation of adhesion force and surface forceat equilibrium condition is explained in detail in Tutuncu A. N., 1992,Velocity Dispersion and Attenuation of Acoustic Waves in GranularSedimentary Media, PhD Dissertation, The University of Texas at Austinand Tutuncu A. N. and Sharma M. M., 1992, The influence of fluids ongrain contact stiffness and frame moduli in sedimentary rocks,Geophysics, V. 57(12), 1571-1582.

${a^{3} = {{\frac{3\; \pi}{4}\left\lbrack {\frac{\left( {1 - v_{1}^{2}} \right)}{\pi \; E_{1}} + \frac{\left( {1 - v_{2}^{2}} \right)}{\pi \; E_{2}}} \right\rbrack}{RF}_{0}}},$

where ν₁ and ν₂ are Poisson's ratios for the spherical particles to beremoved, E₁ and E₂ are the Young's moduli, and R=R₁R₂/(R₁+R₂), F0 isinternal force applied from the Hertz theory. See Timoshenko, S., andGoodier, J. N., 1951, Theory of elasticity: McGraw-Hill Book Co.

FIG. 10 is a graph of the detachment ratio as a function of particlesize for various frequencies for velocity amplitude u_(o)=0.1 cm/sec.FIG. 11 is a graph of the detachment ratio as a function of particlesize for various frequencies for velocity amplitude u_(o)=10 cm/sec.FIG. 12 provides a graph of the detachment ratio as a function ofparticle size for various frequencies for velocity amplitude u_(o)=20cm/sec.

The systems and methods described herein improve the transport andplacement of proppants into fractures within the production interval ofthe wells during the hydraulic fracturing operations. The systems andmethods of embodiments of the present invention enhance production fromwells in conventional and unconventional formations, including tight gassands, shale gas, tight oil, shale oil and others.

As explained herein, prior art acoustic stimulation tools available canonly be used to eliminate some formation damage only very near towellbore. The prior art acoustic stimulation tools are placed in thewell through wireline or tubing and are only useful for eliminatingdamage near the wellbore. The effectiveness of these tools is stronglydependent on the frequency of the acoustic sensors used. Hence, in allprior art acoustic stimulation tools, the utilized frequencies areeffective only up to a few inches from the wellbore resulting incleaning of damaged proppant only very near to the wellbore. None of theprior art tools have been tested to assist proppant packing intofractures or with low frequencies as disclosed in this invention. Priorart tools are also all much less effective than the system and method ofthe present invention as the induced fractures in vertical, inclined andhorizontal wells are typically designed and executed to be tens tohundreds of feet long with multiple stages over thousands of feet longin horizontal wells.

The systems and methods described in this disclosure can be implementedinside the wellbore along the entirety of the production interval withmultiple frequencies. By coupling multiple frequency acoustic waves andpiezoelectric induced mechanical elongation and shortening, the cleaningeffects can propagate through the entire fracture and wellbore networkand can clean damage from the proppant packed well. The fractures can becleaned from the tip of the fractures into the wellbore. Morespecifically, the cleaning effects of the system and method ofembodiments of the present invention are realized for the full length,or a predetermined portion of the length, of the production interval ofa well. Additionally the entire length, or a substantial portion of thelength, of the proppant packed fractures can be cleaned with downholeassembly of embodiments of the present invention. The downhole assemblyof the present invention may be used in wells of any orientation,including horizontal wells, deviated wells, and vertical wells. Proppantin fractures of any type, such as vertical fractures, deviatedfractures, and horizontal fractures can be cleaned using the downholeassembly or piezoelectric crystals pumped together with the proppantsduring the fracture treatment. The fractures can be one or more ofinduced fractures and open natural fractures. This cleaning helps tomaintain the production interval damage-free for the lifecycle of thewell.

Various sizes of piezoelectric crystals can be mixed with the proppantor interconnected on the downhole assembly. The size of thepiezoelectric crystals may be selected and optimized based on one ormore of: (i) the formation and fracture characteristics such as thelength, width and height of the fractures; (ii) the density anddistribution of the natural fractures; (iii) formation characteristicsincluding the compaction properties and strength; (iv) the petrophysicalproperties such porosity, permeability, grain size, pore throat size,formation mineralogy and texture; (v) the composition andcharacteristics of the reservoir fluid; and (vi) the type andpetrophysical characteristics of the proppant pack including theproppant size used for packing, porosity, permeability of the proppantpack, and the fracturing fluid composition and characteristics. Oncepiezoelectric crystals of appropriate sizes, shapes, and frequencies areselected, the piezoelectric crystals can be mixed with the proppant orinterconnected to one or more surfaces of a downhole assembly 16. In oneembodiment, the downhole assembly comprises one or more sections. Thedownhole assembly can be positioned in the well where the proppant packwill be established. When the proppant is pumped into the well, thedownhole assembly will create displacement and/or cavitation duringstarting with the proppant injection during the fracturing process. Thepiezoelectric crystals 28 mixed in the proppant and/or connected to thedownhole assembly 16 will activate (such as by expanding andcontracting) throughout the lifecycle of the well as needed and based onconditions in the well. Utilizing the downhole assembly 16 withpiezoelectric crystals during the fracture stimulation process will aidin efficient proppant placement for an effective proppant pack from thetip of the induced fractures to within the proppant packed wellborealong the horizontal, inclined or vertical well.

During the production phase of the well, rate of production, wellborepressure, temperature near the wellbore region, and the conductivity ofthe proppant pack may be monitored in real time to detect any changes inrelative permeability. Any changes in these monitored parameters willprovide information on permeability decline. Triggering of thepiezoelectric crystals 28 pumped simultaneously together with proppants(such as illustrated in FIGS. 5-8) and/or the piezoelectric crystals 28of a downhole assembly 16 causes mechanical elongation or contraction ofthe piezoelectric crystals. Optionally, the piezoelectric crystals 28may also be activated by an external power source. When shortening ofthe piezoelectric crystals occurs, it also results in cavitation. Thecavitation creates drag forces allowing the cleanup treatment in theproppant pack and in the fracture to recover or improve the permeabilityin the fractures and in the proppant pack, thus improving productionsrates

The piezoelectric crystals 28 can also be deployed with a range ofsweeping frequencies. More specifically, a plurality of piezoelectriccrystals 28 with different frequencies can be selected and positionedwith customized spacing on a downhole assembly. The downhole assemblycan thus be configured to remove any proppant damage induced fromvarious fluids used as fracturing fluids, produced fluids includingproduced water, oil and gas, and fines and particles migrated from theformation into the proppant pack.

The downhole assembly 16 can optionally be reused, once a well is notedto be uneconomic, to reduce costs. The embodiment of the downholeassembly 16A having a body with screens 18 is generally more permanentand is also economical as it provides a lifetime of reliable fracturestimulation eliminating the need for refracturing or other stimulationprocedures. The screen mounted with piezoelectric crystals can bedesigned to provide acoustic cavitation and/or acoustic streaming to aidin fluid flow when deposited near the wellbore region, when the fluidflow is hampered by increased viscosity due to lower temperatures nearthe wellbore.

This invention is not limited to the applications listed above. Morespecifically, the methods and downhole tools of the present inventioncan be used in any type of well which is formed in any geologicformation. Accordingly, the downhole tools and methods described hereincan be utilized in deep-water poorly consolidated high permeabilityreservoir completions. The methods and apparatus can be employed duringthe frac-pack fracture stimulations of the deep-water reservoir. Themethod and apparatus of the present invention will also help ensureeffective gravel packing and would aid in the removal of damage withinthe gravel pack due to the fines and particles migrating from theformation into the gravel pack, the plugging of the pore throats, andthe gas and water blocking seen in offshore operations. Accordingly,embodiments of the invention will increase production by eliminatingproppant pack porosity and permeability damage, and creating aneffective proppant pack.

To provide additional background, context, and to further satisfy thewritten description requirements of 35 U.S.C. § 112, the followingreferences are incorporated by reference herein in their entireties: (1)Beresnev I. A. and Johnson P. A., 1994, Elastic wave stimulation of oilproduction: A review of methods and results: Geophysics, 59(6),1000-1017, available athttps://library.seg.org/doi/abs/10.1190/1.1443645; (2) Birchak J. R.,Ritter T. E., Mese A. I., van Batenburg D., Trainor W., Han W., Yoo K.,Kusmer D., Proett M. A., van der Bas F., van der Sman P., Groenenboom J.and Zuiderwijk P., 2005, Acoustic stimulation method with axial driveractuating moment arms on tines, U.S. Pat. No. 7,216,738; (3) Harthy A.,Abdulkadir R., Sipra I., 2005, Screen and Near-Wellbore Cleaning andStimulation Tools Evaluation: Recent Experience in Well operation, SPE89653, Proc. Coiled Tubing Conference and Exhibition, available athttps://www.onepetro.org/conference-paper/SPE-89653-MS; (4) Malhotra M.,Lehman E. R. and Sharma M. M., 2014, Proppant Placement UsingAlternate-Slug Fracturing, SPE 163851, SPE Journal, V. 19, Issue 5,available at https://www.onepetro.org/journal-paper/SPE-163851-PA; (5)Mese A. I., Soliman M., Robison C., 2005, System and method for treatinga fluid in a pipe, U.S. Patent App. Pub. No. 2005/0161081; (6) Liu Y.,Gadde P. B. and Sharma M. M., 2006, Proppant Placement UsingReverse-Hybrid Frac, SPE 99580, Proc. SPE Gas Technology Symposium,Calgary, Alberta, Canada, available athttps://www.scribd.com/document/334696773/SPE-99580-MS; (7) Soliman M.,Mese A. I., Robison C. E., Birchak J. R., Rodney P. F., Han W., Shah V.V., Linyaev, E. J., Proett M. A., 2003, Method and apparatus fortreating a wellbore with vibratory waves to remove particles therefrom,U.S. Pat. No. 6,619,394 (also published as PCT Pub. WO 2002/046572); (8)Tutuncu A. N. and Mese A. I., 2008, Experimental Investigation ofUltrasonic Cleaning of Drilling and Drill-In Fluids Damage in BereaSandstone Cores, SEG 2008-1640, Proc. SEG Annual Meeting, available athttps://www.onepetro.org/conference-paper/SEG-2008-1640; (9) Tutuncu A.N. and Roha R., 2008, An Experimental Study for Removal of Near-WellboreAsphaltene Deposits Using Ultrasonics, SEG 2008-1719, Proc. SEG AnnualMeeting, available athttps://www.onepetro.org/conference-paper/SEG-2008-1719; (10) Tutuncu,A. N., and M. M. Sharma, 1994, Mechanisms of Colloidal Detachment in aSonic Field: Paper 63e, Proc. 1st AIChE International ParticleTechnology Forum, 24-29; (11) Wong S. W., van der bas F., Zuiderwijk P.,Birchak B., Han W., Yoo K. and van Batenburg D., 2004, High Power/HighFrequency Acoustic Stimulation: A Novel and Effective WellboreStimulation Technology, SPE 84118, SPE Production and FacilitiesJournal, 183-188, available athttps://www.onepetro.org/journal-paper/SPE-84118-PA; (12) Zhou J., SunH., Qu Q., Guerin M. and Li L., 2014, Benefits of Novel Preformed GelFluid System in Proppant Placement for Unconventional Reservoirs, SPE167774, Proc. SPE/EAGE European Unconventional Resources Conference andExhibition, Vienna, Austria, available athttps://www.onepetro.org/conference-paper/SPE-167774-MS; (13) U.S. Pat.No. 5,635,712; (14) U.S. Pat. No. 5,441,110; (15) U.S. Pat. No.6,935,424; (16) U.S. Pat. No. 7,543,635; (17) U.S. Patent Pub.2007/0215345; (18) U.S. Pat. No. 9,695,681; (19) U.S. Patent Pub.2007/0193740; (20) PCT Publication WO2017/105426A1; (21) U.S. Pat. No.8,646,483; (22) U.S. Pat. No. 6,609,067; (23) U.S. Pat. No. 7,653,488;(24) Iriarte J., Katsuki D. and Tutuncu A. N., 2017, FractureConductivity under Triaxial Stress Conditions, Chapter 16 in HydraulicFracture Modeling, Editor Yu-Shu Wu, ISBN 978-0-12-812998-2, ElsevierGulf Professional Publishing, available athttps://www.sciencedirect.com/science/article/pii/B9780128129982000163;(25) Iriarte J., Katsuki D. and Tutuncu A. N., 2018, Geomechanical,Geochemical and Permeability Monitoring in Fractured Niobrara Formationunder Triaxial Stress State, SPE-189839, Proc. SPE Hydraulic FracturingConference, Woodland, Tex., available athttps://www.onepetro.org/conference-paper/SPE-189839-MS; (26) IriarteJ., Katsuki D. and Tutuncu A. N., 2018, Geochemical and GeomechanicalChanges Related to Rock-Fluid-Proppant Interactions in the NiobraraFormation, SPE-189536, Proc. SPE International Conference and Exhibitionon Formation Damage Control, Lafayette, La., available athttps://www.onepetro.org/conference-paper/SPE-189536-MS; (27) Tutuncu A.N., Bui B. T. and Suppachoknirun T., 2017, An Integrated Study forHydraulic Fracture and Natural Fracture Interactions and Re-Fracturingin Shale Reservoirs, Chapter 10 in Hydraulic Fracture Modeling, EditorYu-Shu Wu, ISBN 978-0-12-812998-2, Elsevier Gulf ProfessionalPublishing, available athttps://www.sciencedirect.com/science/article/pii/B9780128129982000102;(28) Tutuncu A. N., 1992, Velocity Dispersion and Attenuation ofAcoustic Waves in Granular Sedimentary Media, PhD Dissertation, TheUniversity of Texas at Austin; (29) Tutuncu A. N. and Sharma M. M.,1992, The influence of fluids on grain contact stiffness and framemoduli in sedimentary rocks, Geophysics, V. 57(12), 1571-1582; (30) U.S.Pat. Pub. 2018/0100389; and (31) U.S. Pat. Pub. 2018/0134950.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimiting of the invention to the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiments described and shown in the figures were chosen and describedin order to best explain the principles of the invention, the practicalapplication, and to enable those of ordinary skill in the art tounderstand the invention.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and alterations of thoseembodiments will occur to those skilled in the art. Moreover, referencesmade herein to “the present invention” or aspects thereof should beunderstood to mean certain embodiments of the present invention andshould not necessarily be construed as limiting all embodiments to aparticular description. It is to be expressly understood that suchmodifications and alterations are within the scope and spirit of thepresent invention, as set forth in the following claims.

What is claimed is:
 1. A downhole assembly for enhancing flow rates froma wellbore, comprising: a body for positioning within a productioninterval of the wellbore; and a plurality of piezoelectric crystalsinterconnected to the body, the piezoelectric crystals havingpredetermined sizes and being interconnected to an exterior surface ofthe body, wherein, when the downhole assembly is positioned within thewellbore, at least one of the plurality of piezoelectric crystalsexpands or contracts in response to at least one of a change intemperature, a change in pressure, and a change in fluid flow rate inthe wellbore.
 2. The downhole assembly of claim 1, wherein the bodyincludes a bore defining an interior surface.
 3. The downhole assemblyof claim 2, wherein at least one piezoelectric crystal is interconnectedto the interior surface of the body.
 4. The downhole assembly of claim1, wherein a first subset of the plurality of piezoelectric crystalsgenerate low frequencies of between approximately 0.1 kHz toapproximately 100 kHz when activated.
 5. The downhole assembly of claim4, wherein a second subset of the plurality of piezoelectric crystalsgenerate high frequencies of between approximately 10 kHz andapproximately 100 MHz when activated.
 6. The downhole assembly of claim5, wherein the first subset of the plurality of piezoelectric crystalsare positioned on the exterior surface of the body and the second subsetof the plurality of piezoelectric crystals are positioned on an interiorsurface of the body.
 7. The downhole assembly of claim 1, wherein thebody comprises at least one of a solid bar and a mesh material.
 8. Thedownhole assembly of claim 1, further comprising a power source toprovide electricity to the plurality of piezoelectric crystals.
 9. Thedownhole assembly of claim 1, further comprising a controller operableto send a signal to activate and deactivate the plurality ofpiezoelectric crystals.
 10. The downhole assembly of claim 1, whereinthe plurality of piezoelectric crystals have sizes and frequenciesselected based on characteristics of the wellbore including at least oneof the depth, length, temperature, flow rate, hydraulic fracturinginterval, reservoir permeability, formation type, and reservoirporosity.
 11. A method of enhancing a flow rate from a wellbore in areservoir, comprising: positioning a downhole assembly in a productioninterval of the wellbore, the downhole assembly including: a body; andpiezoelectric crystals interconnected to the body, a first subset of thepiezoelectric crystals operable to generate low frequencies and a secondsubset of the piezoelectric crystals operable to generate highfrequencies; and triggering at least one of the plurality ofpiezoelectric crystals, wherein the at least one piezoelectric crystalexpands and/or contracts which causes fines in fractures of thereservoir to repair and improve the permeability of a hydraulicreservoir proximate to the wellbore.
 12. The method of claim 11, furthercomprising flowing fluid from the wellbore to flush the fines out of thefractures.
 13. The method of claim 11, wherein the at least onepiezoelectric crystal is triggered by a change in one or more of atemperature, a pressure, and a rate of fluid flow in the wellbore. 14.The method of claim 11, further comprising selecting at least one of apattern, a size, and a frequency of the plurality of piezoelectriccrystals based on a characteristic of the wellbore and the reservoir.15. The method of claim 11, wherein the body comprises a screen or asolid bar for placement in the wellbore.
 16. A method of enhancing theproduction of a hydrocarbon reservoir, comprising: providing a wellboreextending a predetermined length and depth in the hydrocarbon reservoir;providing a fluid which includes a proppant material and a plurality ofpiezoelectric crystals; and pumping the fluid into the wellbore, whereinat least one of the plurality of piezoelectric crystals contracts inresponse to conditions within the wellbore.
 17. The method of claim 16,wherein at least one of the plurality of piezoelectric crystals istransported into a fracture in the hydrocarbon reservoir.
 18. The methodof claim 16, wherein the plurality of piezoelectric crystals generatefrequencies of between approximately 0.1 kHz and approximately 1 GHzwhen activated.
 19. The method of claim 16, wherein the piezoelectriccrystals comprise up to approximately 80% by volume of the fluid. 20.The method of claim 16, wherein the at least one of the plurality ofpiezoelectric crystals contracts in response to a reduction of at leastone of a fluid temperature, a fluid pressure, and a rate of fluid flow.